• 大型動物食用並散播大型硬木（hardwood）的種子
• 狩獵區越來越多的年輕樹木是軟木
• 森林復育計畫亦須將動物數量列為目標

圖片來源：SciDev.Net (Copyright: Penny Tweedie/Panos)

研究人員警告：在熱帶森林過度狩獵大型動物，將會降低大型樹木比例進而減少碳存量。

根據一篇由巴西學者於去年12月在《Science Advances》期刊發表的研究指出，大型動物對於傳播硬木種子極為關鍵。因為硬木可以長得較大，且比起矮較小的軟木，硬木能吸收大氣中更多的二氧化碳。

研究數據來自於巴西東南部的31個森林區，分析2000多種樹種及800多種動物。研究人員模擬了大型攝食水果動物滅絕，包含靈長類、貘、大型鳥類及大型嚙齒動物等，發現這將導致硬木數量衰退。

「硬木可以鎖住較多的碳。」本研究作者之一的英國東英吉利大學（University of East Anglia）生態學家Carlos Peres表示，只有大型動物能夠攝取、消化並散播這些樹木產生的大型果實。

「然而，這些大型動物正在被過度獵殺，而伴隨牠們的消失，大型種子類的樹群也將越來越少。」Peres告訴網站「科學與發展網絡」（SciDev.net），「我們在談論的是一個無聲、不知不覺之間發生的過程。」

因為大型動物消失，森林的組成漸漸地轉為由種子較小的樹種佔上風，而這些樹種傾向是碳儲存能力較差的軟木。它們的種子通常由蝙蝠、小型鳥類、或是嚙齒動物等較少被獵捕的動物傳播。

Peres說，這樣的狀況正發生在所有熱帶地區。泰國、秘魯及西非的森林研究顯示，在狩獵區，較年輕樹木的組成已改變成有比較高比例的輕木（light wood）樹種。

圖片來源：SciDev.Net

英國蘭斯卡特大學（Lancaster University）環境科學家Sam Robinson指出，這個研究顯示，未來的熱帶森林復育計畫，也該納入恢復大型動物的數量。

巴西聖保羅大學（University of Sao Paulo）森林學家Pedro Brancalion則認為，此研究結果將影響現有的計畫，如聯合國的REDD+計畫（Reducing Emission From Deforestation And Degradation，減少毀林及森林退化造成的溫室氣體排放倡議進階版），REDD+致力於保護森林以保持它們的碳儲存容量。

Brancalion說：「這項研究提供了很好的例子：保育或復育森林以減輕氣候變遷時，目光必須超越樹木本身。我們需要更全面性的視野來看待森林退化，以考量其長時間帶來的間接後果。」

這項結果提供在保育或復育森林以調適氣候變遷時，目光需要超越樹木很好的例子。我們需要一個更完整的視野，來看待衰退，必須考量到長時間產生的間接後果。」

參考資料
Carolina Bello and others (2015). Defaunation affects carbon storage in tropical forests（去動物作用對熱帶雨林碳儲存產生的影響）. Science Advances, Vol. 1, no. 11.

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Overhunting chops tropical forest carbon storage

Speed read

• Large animals eat and disperse seeds of large hardwood trees
• More softwood species among young trees in hunting sites
• Forest restoration projects must also target animal populations

Overhunting large animals in tropical forests will reduce the proportion of large trees and reduce carbon storage, researchers warn.

According to a study published in Science Advances on 18 December and led by researchers from Brazil, large animals are crucial for dispersing hardwood tree seeds in tropical forests. These trees grow larger and remove more carbon dioxide from the atmosphere than smaller softwood trees.

The study analysed data from more than 2,000 tree species and 800 animal species in 31 forest areas in southeast Brazil. The researchers simulated the effects of the extinction of large, fruit-eating animals – such as primates, tapirs, large birds and large rodents – showing that it would cause a decline in hardwood species.

“The hard-wooded trees are the ones that lock [away] more carbon,” explains Carlos Peres, an ecologist at the University of East Anglia in the United Kingdom and one of the authors of the paper. He says that only big animals can eat, digest and disperse the large fruit these trees produce.

“However, these large animals are being overhunted and, with their disappearance, the large-seeded tree population will also decrease,” he tells SciDev.Net. “We are talking about a process that is very silent and very insidious.”

Because of large animal loss, forest composition could gradually change as trees with small seeds, which tend to be softwood species that lock away less carbon, become more prevalent. Their seeds are dispersed by bats, small birds and rodents, which are unaffected by hunting, the researchers say.

“This is happening all over the tropics,” says Peres. “Studies of forests in Thailand, Peru and West Africa show that, in hunting sites, the composition of young trees has already been altered, with a higher percentage of light-wooded trees.”

Sam Robinson, an environmental scientist at Lancaster University in the United Kingdom, says the study’s findings show that future projects to restore tropical forest should include efforts to revive the populations of large animals.

The results could also affect existing programmes, such as the UN’s REDD+ initiative, that aim to protect forests to retain their carbon storage capacity, says Pedro Brancalion, a forestry researcher at the University of Sao Paulo in Brazil.

“This study provides a great example of the need to look beyond trees when conserving or restoring tropical forests for mitigating climate change,” says Brancalion. “We need a more holistic overview of degradation, which considers indirect consequences operating [over the] longer term.”

References
Carolina Bello and others. Defaunation affects carbon storage in tropical forests (Science Advances, 18 December 2015)

Written by Barbara Axt
Published on SciDev.Net, December 30, 2015